1. Field of the Invention
The present invention relates to a blower fan and more specifically to a blower fan for use in, e.g., cooling an electronic device.
2. Description of the Related Art
Conventionally, an outer rotor type motor in which a rotor is arranged radially outwards of a stator has been extensively used as a motor for driving a blower fan. From the viewpoint of performance, the outer rotor type motor is greater in inertial moment and drive torque, which makes it easy to keep a rotational speed of the outer rotor type motor constant.
As the density of electronic parts arranged within a housing of an electronic device grows higher in recent years, the electronic device tends to generate an increased amount of heat. Thus, a need exists to increase the rotation speed of a blower fan for cooling the electronic device. However, the outer rotor type motor has a greater moment of inertia. Therefore, if the rotation speed of the blower fan is increased, the blower fan is likely to suffer from unbalanced rotation. This may possibly result in an increased vibration and may possibly pose a problem in terms of strength.
As a solution to this problem, an inner rotor type motor in which a rotor is arranged radially inwards of a stator may be used. Since the inner rotor type motor has a smaller moment of inertia than the outer rotor type motor, it becomes possible to increase the rotational speed of the blower fan.
As one example of a blower fan employing an inner rotor type motor, US2009/0180901A discloses a blower fan including a motor support unit 260 in which a bearing holding unit 261 holding a bearing 220 and a stator holding unit 262 holding a stator 250 are continuously formed into a single piece (see FIG. 11).
In the conventional inner rotor type blower fan disclosed in US2009/0180901, as shown in FIG. 11, a rotor holder 230 (rotor magnets 240) and an impeller cup 270 (blades 280) as rotating bodies are arranged radially outwards of a shaft 210 as a rotation axis, between the bearing holding unit 261, the stator 250 (the stator holding unit 262) and a housing 290 as non-rotating bodies. In the conventional inner rotor type blower fan, therefore, specified spaces need to be provided radially outwards of the rotation axis, between the shaft 210 and the bearing holding unit 261, between the bearing holding unit 261 and the rotor holder 230, between the rotor magnets 240 and the stator 250, between the stator holding unit 262 and the impeller cup 270 and between the blades 280 and the housing 290 as indicated by circles in FIG. 11.
On the other hand, in an outer rotor type blower fan, a bearing holding unit, a stator, a rotor holder (rotor magnets) and an impeller cup (blades) are arranged radially outwards of a rotation axis. The bearing holding unit also serves as a stator holding unit. The impeller cup is press-fitted to the outer circumference of the rotor holder. In the outer rotor type blower fan, therefore, it is only necessary that specified spaces be provided radially outwards of the rotation axis, between the shaft and the bearing holding unit (the stator holding unit), between the stator and the rotor holder (the rotor magnets) and between the blades and the housing.
For the reasons stated above, the spaces to be provided between the non-rotating bodies and the rotating bodies arranged in the radial direction are increased in the inner rotor type blower fan as compared with the outer rotor type blower fan. This results in an increased radial dimension of the inner rotor type blower fan. Accordingly, there is a limit in how much the size of the inner rotor type blower fan can be reduced. In addition, the radial dimension of the impeller cup 270 with respect to the inner diameter of the housing 290 becomes greater which makes it impossible to secure a sufficient wind-tunnel area. As a consequence, it is impossible to sufficiently enhance the cooling capacity despite the increase in the rotation speed of the blower fan.